1. Field of the Invention
The present invention relates to a sheet feeding device that feeds sheets one by one from a repository in which a plurality of sheets are accommodated and to an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
Image forming apparatuses, such as printers and copiers, include a sheet feeding device for supplying sheets one by one to an image forming portion from a repository in which sheets are accommodated. One such example is a sheet feeding device that blows air on an end of a sheet stack accommodated in a repository, raises several sheets, attracts the uppermost sheet of the raised sheets to a attraction conveying belt, and conveys the attracted sheet (hereinafter, this type of sheet feeding device is sometimes referred to as an air-blowing sheet feeding device). This technique is disclosed in Japanese Patent Laid-Open No. 7-196187 (corresponding to U.S. Pat. No. 5,645,274).
One example of a known air-blowing sheet feeding device is described below with reference to FIGS. 15 and 16.
As illustrated in FIG. 15, a repository 11 is provided with a tray 12 supporting sheets S. An attraction conveying portion for attracting and conveying a sheet is disposed above the sheet stack supported on the tray 12. An air blowing portion for raising a plurality of sheets in the upper part of the sheet stack supported on the tray 12 and loosening them is disposed adjacent to a side of the repository 11.
The attraction conveying portion includes a rotatable attraction conveying belt 21 looped around belt driving rollers 41. The attraction conveying belt 21 attracts a sheet and conveys it rightward in FIG. 15. The attraction conveying belt 21 is connected to an attraction mechanism for attracting a sheet. The attraction mechanism includes an attraction fan 36 for producing a negative pressure and a suction duct 51 for sucking air through a suction port formed in the attraction conveying belt 21. The suction duct 51 is connected to the attraction fan 36 and disposed inside the attraction conveying belt 21. An attraction shutter 37 for switching on and off an attraction operation is disposed between the attraction fan 36 and the suction duct 51.
The air blowing portion includes a loosening nozzle 33 and a separation nozzle 34 for blowing air on the upper part of a sheet stack supported on the tray 12 from the side. Air is supplied from a separation fan 31 to the loosening nozzle 33 and the separation nozzle 34 through a separation duct 32. Air drawn from the separation fan 31 in a direction indicated by the arrow C in FIG. 15 is blown in a direction indicated by the arrow D through the loosening nozzle 33 and is also blown in a direction indicated by the arrow E through the separation nozzle 34. The air blown in the direction of the arrow D raises several sheets in the upper part of the sheet stack supported on the tray 12 and loosens them. The air blown in the direction of the arrow E separates the uppermost sheet from the other sheets and attracts it to the attraction conveying belt 21.
To reliably attract the sheets S to the attraction conveying belt 21 one by one, it is necessary to set the uppermost surface of the sheet stack supported on the tray 12 at a height suited for attraction. To this end, an uppermost-sheet detection mechanism 49 (see FIG. 16) for detecting the position of the uppermost surface of the sheet stack supported on the tray 12 is provided. In accordance with detection by the uppermost-sheet detection mechanism 49, the tray 12 is moved upward and downward.
As illustrated in FIG. 16, the uppermost-sheet detection mechanism 49 includes a sensor lever 52 rotatably supported by a support shaft 53 and a sheet surface sensor 54. The leading end of the sensor lever 52 comes into contact with the uppermost surface of the sheet stack and the sensor lever 52 rotates, thus switching on or off the sheet surface sensor 54 and causing the sheet surface sensor 54 to output a signal. In accordance with the signal from the sheet surface sensor 54, a control unit (not shown) controls the upward and downward movements of the tray 12 to set the uppermost surface of the sheet stack at a height suited for attraction.
To feed the sheets S, after the uppermost sheet Sa is moved at a height suited for attraction by movement of the tray 12 upward or downward by use of the uppermost-sheet detection mechanism 49, the uppermost sheet Sa is attracted to the attraction conveying belt 21 by operation of the attraction fan 36. In response to a sheet feed signal Cf output from a printer body, the attraction conveying belt 21 to which the uppermost sheet Sa is attracted is rotated, thus delivering the uppermost sheet Sa to a pair of drawing rollers 42 disposed downstream. After the uppermost sheet Sa passes through the attraction conveying belt 21, the attraction conveying belt 21 is stopped, the upward and downward movements of the tray 12 are controlled by use of the uppermost-sheet detection mechanism 49 to set the next sheet Sb at a height suited for attraction to the attraction conveying belt 21.
In known sheet feed operations, after the rear end of the attracted and conveyed uppermost sheet Sa passes through the attraction conveying belt 21, the height of the next sheet Sb is detected by the uppermost-sheet detection mechanism 49. In accordance with the detection, the upward and downward movements of the tray 12 are controlled.
Unfortunately, such known techniques have the following problems.
The uppermost-sheet detection mechanism 49 detects the height of the sheets S by use of the sensor lever 52 coming into contact with the uppermost surface of the sheets S supported on the tray 12. As illustrated in FIG. 16, during operation of continuously feeding sheets, the sensor lever 52 cannot come into contact with the upper surface of the next sheet Sb to be next attracted until the preceding uppermost sheet Sa has passed through the leading end of the sensor lever 52. Accordingly, the position of the upper surface of the sheet Sb cannot be detected until then.
That is, after the uppermost sheet Sa has passed through the attraction conveying portion, whether the upper surface of the next sheet Sb to be next attracted lies in a predetermined position by use of the uppermost-sheet detection mechanism 49 is determined. In accordance with the determination, the upward and downward movements of the tray 12 are controlled. As a result, when the sheets are not raised smoothly and the raised position of the next sheet Sb is low, the time required for lifting the tray 12 to upwardly move the next sheet Sb to a predetermined range is undesirably increased. The interval between sheets during continuous feed operation is long, and productivity in the image forming portion is decreased. In particular, in the case of a sheet having a heavy basis weight or having a large size, its raised state is unstable. This may significantly decrease productivity.